This invention relates to integrated circuit manufacture and, more particularly, to fabrication of extremely dense very-large-scale integrated circuit (VLSI) designs typically requiring the use of thin-film planarization procedures to produce reliable multilayer interconnect systems. Specifically, the invention is directed to advances in laser planarization of nonrefractory metal films, such as aluminum films, or nonrefractory metallization layers, during the fabrication of multilayer VLSI circuits.
High-performance, large-area integrated circuits often incorporate several layers of interconnect. Planarization processes which smooth and flatten the surface of an integrated circuit at various stages of fabrication are therefore generally employed. The need is particularly acute in the case of wafer-scale integration (e.g., an efficient, high-power wafer-scale integrated circuit can require two, three, or even four layers of interconnect plus two or more ground or power planes). The most severe topographic problems occur in the vicinity of stacked vias, where a connection extends from one interconnect layer to the next layer.
Multilayer interconnect systems for integrated circuits demand one or more planarization procedures, in order to maintain an acceptably flat topography for high-resolution photolithography and for adequate coverage of steps by thin films on higher layers. Traditional approaches have involved planarization of the interlayer insulation (dielectric) layers by smoothing the dielectric between metal layers, either by spin-on application (e.g., polyimide), or by reflow (e.g., phosphosilicate glass), as well as other techniques. In this regard, one dielectric planarization technique uses a scanning CW CO.sub.2 laser to rapidly flow phosphosilicate glass. See Delfino, M., "Phosphosilicate Glass Flow Over Aluminum in Integrated Circuit Devices," I.E.E.E. Elec. Dev. Lett., Vol. EDL-4, No. 3, 1983, pp. 54-56. However, planarization of the insulating layers does not by itself provide a fully planar multilayer interconnect process. Severe stepcoverage problems still occur where metals are deposited over deep vertical vias in an insulator, and this problem is accentuated if vias are vertically stacked. None of these dielectric planarization techniques can planarize a deep vertical or stacked (nested) via, because the dielectric must be removed from the contact area between each layer, resulting in a large thickness deficiency at the via.
Celler, et al., U.S. Pat. No. 4,258,078, disclose using an Nd:YAG laser or electron beam to eliminate sharp features that appear on metallization patterns defined by conventional etching processes, to aid in the exposure of defects in conductor rails, that eventuate in open circuits; to repair filamentary shorts between conductor rails; or, in the case of refractory metals, such as polysilicon, molybdenum, tungsten, etc., to improve conductivity. This patent relates to a process which would apply after the metal was planarized and patterned and to refractory, as opposed to nonrefractory, metals. In any event, the metals used for metallization are melted and quenched in a period so short that surface tension acts on the shape of the metal, but flow does not occur, thereby precluding planarization of the metal.
An alternative approach to achieve planarity in multilayer interconnect systems involves actual planarization of metal layers. One metal planarization process is RF bias sputtering. See Mogami, T., Okabayashi, H., Nagasawa, E., and Morimoto, M., "Planarized Molybdenum Interconnection Using Via-Hole Filling by Bias Sputtering," Proc. 1985 VLSI Multilevel Interconnection Conf. (V-MIC), I.E.E.E. Cat. 85CH2197-2, June, 1985, pp. 17-23. However, this metal planarization process relates to refractory metals.
In contrast, a nonrefractory metal, such as gold or aluminum, can be melted using rapid thermal annealing techniques. The disadvantage of these metal planarization techniques is the length of time that the metal remains molten. Unacceptable metallurgical reactions can be induced in thin films (e.g., molten aluminum reduces an SiO.sub.2 dielectric to form silicon and volatile suboxides in a few seconds, and, based on typical thermal diffusivities in molten metals, 1 micrometer of molten gold can completely alloy with a titanium adhesion layer in about 1 ms). This causes device degradation.
Additionally, Tuckerman, D. B., and Schmitt, R. L., "Pulsed Laser Planarization of Metal Films For Multilevel Interconnects," Proc. 1985 VLSI Multilevel Interconnection Conf. (V-MIC), I.E.E.E. Cat. 85CH2197-2, June, 1985, pp. 24-31, disclose that each metal layer can be melted (hence planarized) using a pulsed laser prior to patterning. Planarization of gold films is achieved (less than 0.1 micrometer surface roughness, even starting with extreme topographic variations); and conductivity is also improved. However, for the Au/SiO.sub.2 structure disclosed in the article, an adhesion layer, such as Cr, is necessary at every interface between the two materials.
Furthermore, the article discloses planarization of gold films on SiO.sub.2 dielectric layers using a linear flashlamp-pumped pulsed dye laser containing a coumarin dye, supplying optical pulses having a wavelength of 504 nm. The pulses have a 1 microsecond duration (full width at half maximum), 150 mJ of energy, and 1 Hz repetition rate. Unfortunately, the authors primarily utilized an unstable dye laser, and there is limited throughput due to the shortcomings of the dye laser.
The article additionally indicates that if the duration of the melt is much longer than 1 microsecond, unacceptable metallurgical reactions can be induced in thin films. A few experiments were performed using a KrF excimer laser (248 nm) as the pulse source, but the 10 ns pulse provided a substantially reduced operating window between melting and damage, compared with the factor-of-two window available with the 1 microsecond pulsed dye laser. High incident optical pulse energies produce large crystal grain sizes, which more closely approach theoretical conductivity. Accordingly, a high energy optical pulse of short duration to reflow the metal would be desirable, since conductivity could be improved, but deleterious metallurgical reactions would still be avoided.
Also, in order that sufficient metal area is molten at one time, the article discloses that the laser beam is focused to a 2 mm diameter spot on the wafer and is used to melt a large area (4 mm.sup.2) of metal with a single pulse. However, the theoretical energies needed to be more than doubled to account for the approximately 50% reflectance of the gold and to allow for cold spots in the laser beam. No effort was made to make the spot uniform in intensity, and, consequently, about one-half the pulse energy (around the beam circumference) was below the melt threshold and hence wasted.
Furthermore, the article notes that the metal film should be able to absorb a significant amount of the incident optical power. For this reason, the authors substantially avoided planarizing highly reflective metals, such as aluminum and silver, and instead planarized gold. This is unfortunate because of the wide usage of aluminum in the industry.
In this regard, gold has greater than 50% absorbance in the green or blue region of the spectrum (48% of the energy being reflected by gold films) and has no native oxide. Laser planarization of aluminum films presents a more difficult problem. The high reflectance of aluminum (approximately 92% for visible light) requires high optical pulse energy. Unfortunately, small variations in surface texture, topography, and composition can significantly increase the absorbed power, causing damage. Aluminum's refractory native oxide also presents a serious problem, for it typically remains as a solid skin, impeding planarization.
The article indicates that the wafers were exposed to ambient air during the planarization process. The authors report that no adverse effects appear to be associated with this procedure, and indicate that in the case of aluminum, which oxidizes easily, while there might be reason to operate in an oxygen-free environment, satisfactory results were apparently obtained without doing so.
Nevertheless, at least one of the authors in a later article proposes that one can instead overcoat reflective metal films, such as aluminum, with a thin absorbing layer. See Tuckerman, D. B., and Weisberg, A. H., "Planarization of Gold and Aluminum Thin Films Using a Pulsed Laser," I.E.E.E. Elec. Dev. Lett., Vol. EDL-7, No. 1, January, 1986, pp. 1-4. This article discloses sputter-depositing a thin (&lt;200-Angstrom) layer of amorphous silicon over the aluminum after sputter-etching off the native oxide, which passivates the aluminum against oxide formation prior to laser planarization and also acts as an antireflection coating to increase its initial optical absorbance. While this greatly aids the aluminum planarization process, the requirement of a silicon overcoat to consistently planarize aluminum films complicates the fabrication process. Furthermore, the planarized aluminum films are stressed, which can lead to device failure.